The rate of chemical reactions. The rate of a chemical reaction and the factors influencing it. What does the rate of a reaction in chemistry depend on?

Sections: Chemistry

The purpose of the lesson

• educational: continue to formulate the concept of “rate of chemical reactions”, derive formulas for calculating the rate of homogeneous and heterogeneous reactions, consider on what factors the rate of chemical reactions depends;
• developing: learn to process and analyze experimental data; be able to find out the relationship between the rate of chemical reactions and external factors;
• educational: continue the development of communication skills during pair and group work; focus students' attention on the importance of knowledge about speed chemical reaction occurring in everyday life (metal corrosion, milk souring, rotting, etc.)

Teaching aids: D. multimedia projector, computer, slides on the main issues of the lesson, CD “Cyril and Methodius”, tables on the tables, laboratory reports, laboratory equipment and reagents;

Teaching methods: reproductive, research, partially search;

Form of organization of classes: conversation, practical work, independent work, testing;

Form of organization of student work: frontal, individual, group, collective.

1. Class organization

Readiness of the class for work.

2. Preparation for the main stage of mastering educational material. Activation of basic knowledge and skills(Slide 1, see presentation for the lesson).

The topic of the lesson is “Rate of chemical reactions. Factors influencing the rate of a chemical reaction."

Task: find out what the rate of a chemical reaction is and what factors it depends on. During the lesson, we will get acquainted with the theory of the issue on the above topic. In practice, we will confirm some of our theoretical assumptions.

Predicted student activities

The active work of students shows their readiness to perceive the topic of the lesson. Students need knowledge about the rate of chemical reactions from the 9th grade course (intra-subject communication).

Let's discuss the following questions (frontal, slide 2):

1. Why do we need knowledge about the rate of chemical reactions?
2. What examples can confirm that chemical reactions occur at different rates?
3. How is speed determined? mechanical movement? What is the unit of measurement for this speed?
4. How is the rate of a chemical reaction determined?
5. What conditions must be created for a chemical reaction to begin?

Let's look at two examples (the teacher conducts the experiment).

On the table there are two test tubes, in one there is an alkali solution (KOH), in the other there is a nail; Pour CuSO4 solution into both test tubes. What are we observing?

Predicted student activities

Using examples, students judge the speed of reactions and draw appropriate conclusions. Record the reactions performed on the board (two students).

In the first test tube the reaction occurred instantly, in the second there were no visible changes yet.

Let's create reaction equations (two students write equations on the board):

1. CuSO 4 + 2KOH = Cu(OH) 2 + K 2 SO 4 ;
2. Cu 2+ + 2OH - = Cu(OH) 2

Fe + CuSO 4 = FeSO 4 + Cu; Fe 0 + Cu 2+ = Fe 2+ + Cu 0 What conclusion can we draw from the reactions performed? Why does one reaction occur instantly, the other slowly? To do this, it is necessary to remember that there are chemical reactions that occur throughout the entire volume of the reaction space (in gases or solutions), and there are others that occur only on the contact surface of substances (combustion solid

Predicted student activities

in a gas, the interaction of a metal with an acid, a salt of a less active metal). Based on the results of the demonstrated experiment, students conclude:

reaction 1 is homogeneous, and reaction

2 – heterogeneous.

The rates of these reactions will be mathematically determined in different ways. The study of the rates and mechanisms of chemical reactions is called

chemical kinetics. 3. Assimilation of new knowledge and methods of action

(Slide 3)

The rate of a reaction is determined by the change in the amount of substance per unit time

In unit V

(for homogeneous)

Per unit surface of contact of substances S (for heterogeneous)

Predicted student activities

Obviously, with this definition, the reaction rate does not depend on the volume in a homogeneous system and on the contact area of ​​the reagents in a heterogeneous system.

Active actions of students with the object of study. Entering the table into a notebook. Two things follow from this important points

(slide 4):

2) the calculated value of the speed will depend on the substance by which it is determined, and the choice of the latter depends on the convenience and ease of measuring its quantity.

For example, for the reaction 2H 2 + O 2 = 2H 2 O: υ (by H 2) = 2 υ (by O 2) = υ (by H 2 O)

4. Consolidation of primary knowledge about the rate of chemical reaction

Predicted student activities

To consolidate the material considered, let’s solve a calculation problem.

Primary comprehension of the acquired knowledge about reaction speed. Correctness of the problem solution. Task The chemical reaction occurs in solution according to the equation: A + B = C. Initial concentrations: substance A - 0.80 mol/l, substance B - 1.00 mol/l. After 20 minutes, the concentration of substance A decreased to 0.74 mol/l. Determine: a) the average reaction rate for this period of time;

b) concentration of substance B after 20 minutes. Solution (Appendix 4, slide 6).

5. Assimilation of new knowledge and methods of action(conducting laboratory work during repetition and study of new material, step by step, Appendix 2).

We know that the rate of a chemical reaction is influenced by various factors. Which?

Predicted student activities

Reliance on the knowledge of grades 8-9, recording in notebooks as you study the material. They list (slide 7):

The nature of the reacting substances;

Temperature;

Concentration of reactants;

Action of catalysts;

Contact surface of reacting substances (in heterogeneous reactions).

The influence of all these factors on the reaction rate can be explained using simple theory – collision theory (slide 8). Its main idea is this: reactions occur when particles of reactants that have a certain energy collide.

From this we can draw conclusions:

1. The more reactant particles there are, the closer they are to each other, the more likely they are to collide and react.
2. They only lead to a reaction effective collisions, those. those in which “old connections” are destroyed or weakened and therefore “new ones” can be formed. But for this, the particles must have sufficient energy.

The minimum excess energy (above the average energy of particles in the system) required for effective collision of particles in the system) required for effective collision of particles of reagents is calledactivation energy E A.

Predicted student activities

Understanding the concept and writing the definition in a notebook.

Thus, on the path of all particles entering the reaction, there is a certain energy barrier equal to the activation energy. If it is small, then there are many particles that successfully overcome it. With a large energy barrier, additional energy is needed to overcome it, sometimes a good “push” is enough. I light a spirit lamp - I impart additional energy E A, necessary to overcome the energy barrier in the reaction between alcohol molecules and oxygen molecules.

Let's consider factors, which affect the speed of the reaction.

1) Nature of the reacting substances(slide 9). The nature of reacting substances is understood as their composition, structure, mutual influence of atoms in inorganic and organic substances.

The magnitude of the activation energy of substances is a factor through which the influence of the nature of the reacting substances on the reaction rate is affected.

Briefing.

Independent formulation of conclusions (Appendix 3 at home)

The mechanisms of chemical transformations and their rates are studied by chemical kinetics. Chemical processes occur over time at different rates. Some happen quickly, almost instantly, while others take a very long time to occur.

In contact with

Speed ​​reaction- the rate at which reagents are consumed (their concentration decreases) or reaction products are formed per unit volume.

Factors that can influence the rate of a chemical reaction

The following factors can affect how quickly a chemical reaction occurs:

• concentration of substances;
• nature of reagents;
• temperature;
• presence of a catalyst;
• pressure (for reactions in a gas environment).

Thus, by changing certain conditions of a chemical process, you can influence how quickly the process will proceed.

In progress chemical interaction particles of reacting substances collide with each other. The number of such coincidences is proportional to the number of particles of substances in the volume of the reacting mixture, and therefore proportional to the molar concentrations of the reagents.

Law of mass action describes the dependence of the reaction rate on the molar concentrations of the substances that interact.

For an elementary reaction (A + B → ...) this law is expressed by the formula:

υ = k ∙С A ∙С B,

where k is the rate constant; C A and C B are the molar concentrations of reagents A and B.

If one of the reacting substances is in a solid state, then the interaction occurs at the interface; therefore, the concentration of the solid substance is not included in the equation of the kinetic law of mass action. To understand the physical meaning of the rate constant, it is necessary to take C, A and C B equal to 1. Then it becomes clear that the rate constant is equal to the reaction rate at reactant concentrations equal to unity.

Nature of the reagents

Since during the interaction the chemical bonds of the reacting substances are destroyed and new bonds of the reaction products are formed, the nature of the bonds involved in the reaction of the compounds and the structure of the molecules of the reacting substances will play a large role.

Surface area of ​​contact of reagents

Such a characteristic as the surface area of ​​contact of solid reagents affects the course of the reaction, sometimes quite significantly. Grinding a solid allows you to increase the surface area of ​​​​contact of the reagents, and therefore speed up the process. The contact area of ​​soluble substances is easily increased by dissolving the substance.

Reaction temperature

As the temperature increases, the energy of colliding particles will increase; it is obvious that with increasing temperature the chemical process itself will accelerate. A clear example of how an increase in temperature affects the process of interaction of substances can be considered the data given in the table.

Table 1. Effect of temperature changes on the rate of water formation (O 2 +2H 2 →2H 2 O)

To quantitatively describe how temperature can affect the rate of interaction of substances, the Van't Hoff rule is used. Van't Hoff's rule is that when the temperature increases by 10 degrees, an acceleration occurs by 2-4 times.

The mathematical formula describing van't Hoff's rule is as follows:

Where γ is the temperature coefficient of the rate of the chemical reaction (γ = 2−4).

But the Arrhenius equation describes the temperature dependence of the rate constant much more accurately:

Where R is the universal gas constant, A is a factor determined by the type of reaction, E, A is the activation energy.

Activation energy is the energy that a molecule must acquire for a chemical transformation to occur. That is, it is a kind of energy barrier that molecules colliding in the reaction volume will need to overcome in order to redistribute bonds.

The activation energy does not depend on external factors, but depends on the nature of the substance. The activation energy value of up to 40 - 50 kJ/mol allows substances to react with each other quite actively. If the activation energy exceeds 120 kJ/mol, then the substances (at ordinary temperatures) will react very slowly. A change in temperature leads to a change in the number of active molecules, that is, molecules that have reached an energy greater than the activation energy, and therefore are capable of chemical transformations.

Catalyst action

A catalyst is a substance that can speed up a process, but is not part of its products. Catalysis (acceleration of a chemical transformation) is divided into homogeneous and heterogeneous. If the reactants and catalyst are in the same states of aggregation, then catalysis is called homogeneous, if different, then heterogeneous. The mechanisms of action of catalysts are varied and quite complex. In addition, it is worth noting that catalysts are characterized by selectivity of action. That is, the same catalyst, while accelerating one reaction, may not change the rate of another.

Pressure

If gaseous substances are involved in the transformation, then the rate of the process will be affected by changes in pressure in the system . This happens because that for gaseous reagents, a change in pressure leads to a change in concentration.

Experimental determination of the rate of a chemical reaction

The speed of a chemical transformation can be determined experimentally by obtaining data on how the concentration of substances entering the reaction or products changes per unit time. Methods for obtaining such data are divided into

• chemical,
• physico-chemical.

Chemical methods are quite simple, accessible and accurate. With their help, the speed is determined by directly measuring the concentration or amount of the substance of the reactants or products. In case of a slow reaction, samples are taken to monitor how the reagent is consumed. Then the content of the reagent in the sample is determined. By taking samples at regular intervals, it is possible to obtain data on changes in the amount of a substance during the interaction process. The most commonly used types of analysis are titrimetry and gravimetry.

If the reaction proceeds quickly, then it has to be stopped in order to take a sample. This can be done using cooling, abrupt removal of the catalyst, it is also possible to dilute or transfer one of the reagents to a non-reactive state.

Methods of physicochemical analysis in modern experimental kinetics are used more often than chemical ones. With their help, you can observe changes in the concentrations of substances in real time. In this case, there is no need to stop the reaction and take samples.

Physicochemical methods are based on measurement physical properties, depending on the quantitative content of a certain compound in the system and changing over time. For example, if gases are involved in a reaction, then pressure may be such a property. Electrical conductivity, refractive index, and absorption spectra of substances are also measured.

Question 1. What substances are called catalysts?

Substances that change the rate of a chemical reaction while remaining unchanged at the end are called catalysts.

Question 2. What role do enzymes play in the cell?

Enzymes are biological catalysts that accelerate chemical reactions in a living cell. The molecules of some enzymes consist only of proteins, others include protein and a compound of a non-protein nature (organic - coenzyme or inorganic - ions of various metals). Enzymes are strictly specific: each enzyme catalyzes a specific type of reaction that involves certain types of substrate molecules.

Question 3. On what factors can the rate of enzymatic reactions depend?

The rate of enzymatic reactions largely depends on the concentration of the enzyme, the nature of the substance, temperature, pressure, reaction of the medium (acidic or alkaline).

For many enzymes, under certain conditions, for example in the presence of molecules of certain substances, the configuration of the active center changes, which allows them to provide the greatest enzymatic activity.

Question 4. Why do most enzymes lose their catalytic properties at high temperatures?

High environmental temperatures, as a rule, cause denaturation of the protein, i.e., disruption of its natural structure. Therefore, at high temperatures, most enzymes lose their catalytic properties.

Question 5. Why can a lack of vitamins cause disturbances in the body’s vital processes?

Many vitamins are part of enzymes. Therefore, a lack of vitamins in the body leads to a weakening of enzyme activity in cells, and therefore can cause disturbances in vital processes.

1.8. Biological catalysts

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§ 12. KINETICS OF ENZYMATIVE REACTIONS

Kinetics of enzymatic reactions is the science of the rates of enzymatic reactions and their dependence on various factors. The rate of an enzymatic reaction is determined by the chemical amount of the reacted substrate or the resulting reaction product per unit time per unit volume under certain conditions:

where v is the rate of the enzymatic reaction, is the change in the concentration of the substrate or reaction product, t is time.

The rate of an enzymatic reaction depends on the nature of the enzyme, which determines its activity. The higher the enzyme activity, the faster the reaction rate. Enzyme activity is determined by the rate of reaction catalyzed by the enzyme. The measure of enzyme activity is one standard unit of enzyme activity. One standard unit of enzyme activity is the amount of enzyme that catalyzes the conversion of 1 µmol of substrate in 1 minute.

During an enzymatic reaction, the enzyme (E) interacts with the substrate (S), resulting in the formation of an enzyme-substrate complex, which then disintegrates to release the enzyme and product (P) of the reaction:

The speed of the enzymatic reaction depends on many factors: the concentration of the substrate and enzyme, temperature, pH of the environment, the presence of various regulatory substances that can increase or decrease the activity of enzymes.

Interesting to know! Enzymes are used in medicine to diagnose various diseases. During myocardial infarction, due to damage and breakdown of the heart muscle, the content of the enzymes aspartate transaminase and alanine aminotransferase in the blood increases sharply. Detection of their activity makes it possible to diagnose this disease.

Effect of substrate and enzyme concentration on the rate of enzymatic reaction

Let's consider the effect of substrate concentration on the rate of the enzymatic reaction (Fig. 30). At low concentrations of the substrate, the rate is directly proportional to its concentration; then, as the concentration increases, the reaction rate increases more slowly, and at very high concentrations of the substrate, the rate is practically independent of its concentration and reaches its maximum value (V max). At such substrate concentrations, all enzyme molecules are part of the enzyme-substrate complex, and complete saturation of the active centers of the enzyme is achieved, which is why the reaction rate in this case is practically independent of the substrate concentration.

Rice. 30. Dependence of the speed of an enzymatic reaction on the concentration of the substrate

The graph of the dependence of enzyme activity on substrate concentration is described by the Michaelis–Menten equation, which received its name in honor of the outstanding scientists L. Michaelis and M. Menten, who made a great contribution to the study of the kinetics of enzymatic reactions,

where v is the rate of the enzymatic reaction; [S] – substrate concentration; K M – Michaelis constant.

Let us consider the physical meaning of the Michaelis constant. Provided that v = ½ V max , we obtain K M = [S]. Thus, the Michaelis constant is equal to the substrate concentration at which the reaction rate is half the maximum.

The rate of the enzymatic reaction also depends on the concentration of the enzyme (Fig. 31). This dependence is straightforward.

Rice. 31. Dependence of the speed of an enzymatic reaction on the concentration of the enzyme

Effect of temperature on the rate of enzymatic reaction

The dependence of the enzymatic reaction rate on temperature is shown in Fig. 32.

Rice. 32. Dependence of the rate of enzymatic reaction on temperature.

At low temperatures (up to approximately 40 - 50 o C), an increase in temperature for every 10 o C in accordance with Van't Hoff's rule is accompanied by an increase in the rate of chemical reaction by 2 - 4 times. At high temperatures above 55 - 60 o C, the activity of the enzyme sharply decreases due to its thermal denaturation, and, as a consequence of this, a sharp decrease in the rate of the enzymatic reaction is observed. Maximum enzyme activity is usually observed within the range of 40 - 60 o C. The temperature at which enzyme activity is maximum is called the temperature optimum. The temperature optimum for enzymes of thermophilic microorganisms is in the region of higher temperatures.

Effect of pH on the rate of enzymatic reaction

The dependence of enzymatic activity on pH is shown in Fig. 33.

Rice. 33. The influence of pH on the rate of enzymatic reaction

The graph of pH is bell-shaped. The pH value at which enzyme activity is maximum is called pH optimum enzyme. The pH optimum values ​​for various enzymes vary widely.

The nature of the dependence of the enzymatic reaction on pH is determined by the fact that this indicator affects:

a) ionization of amino acid residues involved in catalysis,

b) ionization of the substrate,

c) conformation of the enzyme and its active center.

Enzyme inhibition

The rate of an enzymatic reaction can be reduced by a number of chemical substances, called inhibitors. Some inhibitors are poisons for humans, for example, cyanide, others are used as medicines.

Inhibitors can be divided into two main types: irreversible And reversible. Irreversible inhibitors (I) bind to the enzyme to form a complex, the dissociation of which with restoration of enzyme activity is impossible:

An example of an irreversible inhibitor is diisopropyl fluorophosphate (DFP). DPP inhibits the enzyme acetylcholinesterase, which plays an important role in the transmission of nerve impulses. This inhibitor interacts with serine in the active center of the enzyme, thereby blocking the activity of the latter. As a result, the ability of the processes of nerve cells of neurons to conduct nerve impulses is impaired. DPP is one of the first nerve agents. Based on it, a number of products that are relatively non-toxic for humans and animals have been created. insecticides - substances poisonous to insects.

Reversible inhibitors, unlike irreversible ones, can be easily separated from the enzyme under certain conditions. The activity of the latter is restored:

Reversible inhibitors include competitive And non-competitive inhibitors.

A competitive inhibitor, being a structural analogue of the substrate, interacts with the active center of the enzyme and thus blocks the substrate's access to the enzyme. In this case, the inhibitor does not undergo chemical transformations and binds to the enzyme reversibly. After dissociation of the EI complex, the enzyme can contact either the substrate and convert it, or an inhibitor (Fig. 34.). Since both the substrate and the inhibitor compete for space at the active site, this inhibition is called competitive.

Rice. 34. Mechanism of action of a competitive inhibitor.

Competitive inhibitors are used in medicine. Sulfonamide drugs were previously widely used to combat infectious diseases. They are close in structure to para-aminobenzoic acid(PABA), an essential growth factor for many pathogenic bacteria. PABA is the predecessor folic acid, which serves as a cofactor for a number of enzymes. Sulfonamide drugs act as a competitive inhibitor of enzymes for the synthesis of folic acid from PABA and thereby inhibit the growth and reproduction of pathogenic bacteria.

Noncompetitive inhibitors are not structurally similar to the substrate and, when EI is formed, they interact not with the active center, but with another site of the enzyme. The interaction of the inhibitor with the enzyme leads to a change in the structure of the latter. The formation of the EI complex is reversible, therefore, after its breakdown, the enzyme is again able to attack the substrate (Fig. 35).

Rice. 35. Mechanism of action of a non-competitive inhibitor

Cyanide CN - can act as a non-competitive inhibitor. It binds to metal ions that are part of prosthetic groups and inhibits the activity of these enzymes. Cyanide poisoning is extremely dangerous. They can be fatal.

Allosteric enzymes

The term “allosteric” comes from the Greek words allo - other, stereo - site. Thus, allosteric enzymes, along with the active center, have another center called allosteric center(Fig. 36). Substances that can change the activity of enzymes bind to the allosteric center; these substances are called allosteric effectors. Effectors are positive - activating the enzyme, and negative - inhibiting, i.e. reducing enzyme activity. Some allosteric enzymes can be affected by two or more effectors.

Rice. 36. Structure of an allosteric enzyme.

Regulation of multienzyme systems

Some enzymes act in concert, combining into multienzyme systems in which each enzyme catalyzes a specific stage of the metabolic pathway:

In a multienzyme system, there is an enzyme that determines the rate of the entire sequence of reactions. This enzyme is usually allosteric and is located at the beginning of the metabolite pathway. It is capable, by receiving various signals, of both increasing and decreasing the rate of the catalyzed reaction, thereby regulating the speed of the entire process.